fix(ci): use GITHUB_TOKEN for GHCR auth on staging server

Match the working pattern from cd-staging.yml instead of requiring
a separate GHCR_PAT secret. GITHUB_TOKEN is automatically available
in GitHub Actions and has the necessary permissions for ghcr.io.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/cd-staging-tagged.yml

@@ -309,7 +309,7 @@ jobs:
       - name: Login to GHCR on staging server
         run: |
           ssh ${{ secrets.STAGING_USER }}@${{ secrets.STAGING_HOST }} << EOF
-            echo "${{ secrets.GHCR_PAT }}" | docker login ghcr.io -u ${{ github.repository_owner }} --password-stdin
+            echo "${{ secrets.GITHUB_TOKEN }}" | docker login ghcr.io -u ${{ github.actor }} --password-stdin
           EOF
 
       - name: Deploy service